AppleInsider is reporting that customers who want their iPhone service pre-paid may be paying more than $600 for the device.

A report from Piper Jaffray analyst, Gene Munster,  estimates that 53 percent of the new iPhone 3G’s market is made up of customers who forgo service agreements and go the pre-paid route.  But, if they want the new iPhone 3G they’ll be paying a hefty price.

As Apple charges an average of $425 per handset, the estimate assumes that carriers will tack on another $175 or so to make a profit.

With a price more than three-times the subsidized price, it is estimated that only 10-15% of iPhone 2.0 users, mostly international customers, will go with a pre-paid plan.

AT&T has yet to announce an unsubsidized price for the iPhone 3G in the United States.